why has my Samsung Galaxy S6 started an unauthorised download??

I plugged my phone into the charger then the screen went blue with a download symbol and saying
"download.."
"do not turn off target" in the middle of the screen and also small writing in the top left corner saying
"factory mode"
"product name: SM-G920I"
"current binary: Samsung official"
"system status: official"
"Reactivation Lock: OFF"
"Secure download: enabled"
"knox warranty void: 0(0x0000)"
"ap swerve: 8:3 K:2 S:2"